fastboot: Enable flashing by default on sunxi

Now that more and more devices are built using eMMC, providing a way to
easily flash the system without too much hassle seems like a right thing to
do.

Since fastboot is the most deployed tool to do that these days, we can just
rely on it to provide a way to flash the various components in the system
(SPL, U-Boot and the system itself) easily, especially since you can upload
the U-Boot hosting the fastboot "server" through FEL.
Reviewed-by: NAndre Przywara <andre.przywara@arm.com>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>

cmd/fastboot/Kconfig

@@ -66,6 +66,7 @@ config FASTBOOT_USB_DEV
 
 config FASTBOOT_FLASH
 	bool "Enable FASTBOOT FLASH command"
+	default y if ARCH_SUNXI
 	help
 	  The fastboot protocol includes a "flash" command for writing
 	  the downloaded image to a non-volatile storage device. Define